1. Field of the Invention
The present invention relates to a nanoemulsion including at least one nonionic and/or anionic amphiphilic lipid and at least one neutral water-soluble polymer, and to the use of the said nanoemulsion in topical application, in particular in the cosmetic and dermatological fields and in the pharmaceutical and/or ophthalmological fields.
2. Discussion of the Background
Oil-in-water (O/W) emulsions are well known in the cosmetic and dermatological field, in particular for the preparation of cosmetic products such as milks, creams, tonics, serums and toilet waters.
Nanoemulsions are O/W emulsions wherein the size of the oily globules is less than 100 nm, and wherein the oily globules are stabilized by a wreath of amphiphilic lipids, which can optionally form a liquid crystal phase of lamellar type, situated at the oil/aqueous phase interface. The transparency of nanoemulsions originates from the small size of the oily globules, and the small size is obtained by virtue of mechanical energy and in particular the use of a high-pressure homogenizer during preparation. Nanoemulsions are distinguished from microemulsions by their structure. Microemulsions are thermodynamically stable dispersions composed of amphiphilic lipid micelles, which are swollen by oil. In contrast to nanoemulsions, microemulsions do not require high mechanical energy to be prepared; they are formed spontaneously by simply bringing the constituents into contact. The major disadvantages of microemulsions relate to their high proportion of surfactants, which result in intolerances and lead to a sticky feel when applied to the skin. The range of microemulsion formulation is generally very narrow, and their temperature stability is very limited.
Nanoemulsions include one or more amphiphilic lipid(s). The term “amphiphilic lipid” is understood to mean here any molecule having a bipolar structure, that is to say including at least one hydrophobic part and at least one hydrophilic part, and having the property of reducing the surface tension of water (γ<55 mN/m) and of reducing the interfacial tension between water and an oily phase. Synonyms for amphiphilic lipid include, for example: surfactant, surface-active agent, emulsifier.
EP-A-728 460 and EP-A-780 114 disclose nanoemulsions based on liquid nonionic amphiphilic lipids or on silicone surfactants. Nanoemulsions are also disclosed in FR-A-2 787 026, FR-A-2 787 027, FR-A-2 787 325, FR-A-2 787 326, FR-A-2 787 703 and FR-A-2 787 728.
In order for the nanoemulsions as disclosed in the above references to be used as milks or creams and in particular in the care field, they have to be rendered thicker and thus their viscosity has to be increased. There are two conventional ways to increase the viscosity of a nanoemulsion. The first way is to increase the fraction of the dispersed oily phase. This is because it is generally found that, from 22% by weight of oily phase with respect to the total weight of the composition, the viscosity increases as a function of the level of oil. This method, disclosed in the above-mentioned applications, makes it possible to obtain thick, transparent and stable compositions. However, such a method for thickening nanoemulsions is constrained by the need to have a high level of oil, which is not always desired as the formulas obtained are richer (high level of fatty phase), and the viscosity range is narrower.
The second way to increase the viscosity of a nanoemulsion is to add to the nanoemulsion a water-soluble polymer, which, by gelling of the continuous aqueous phase, will increase the viscosity of the combined mixture, even with low levels of oil. In the above-mentioned applications, the addition is envisaged of water-soluble polymers such as hydroxypropyl cellulose, algal derivatives, natural gums and synthetic polymers such as polymers and copolymers of carboxyvinyl acids, for example Carbopols which are anionic polymers. Unfortunately, the addition of such polymers affects the transparency of the products obtained; or else to keep the transparency, a small amount of polymer has to be introduced, which limits their effect in thickening the composition.
Thus, the need remains for a thickening system which makes it possible to suitably thicken a composition in the form of an oil-in-water nanoemulsion without influencing the cosmetic properties of the said compositions, in particular without influencing the transparent nature of the nanoemulsion, whatever the level of oil which it is desired to use.